Carbon nanotubes (CNT) are nanometer-scale sized tube-shaped molecules having the structure of a graphite molecule rolled into a rube. A nanotube can be single-walled or multi-walled, dependent upon conditions of preparation. Carbon nanotubes typically are electrically conductive and mechanically strong and stiff along their length. Nanotubes typically also have a relatively high aspect ratio (length/diameter ratio). Due to these properties, the use of CNTs as reinforcements in composite materials for both structural and functional applications would be advantageous.
However, there are several drawbacks associated with carbon nanotube-reinforced composites. First, CNTs are known to be extremely expensive due to the low yield and low production and purification rates commonly associated with all of the current CNT preparation processes. The high material costs have significantly hindered the widespread application of CNTs. Second, it is well-known in the field of composites that the reinforcement fiber orientation plays an important role in governing the mechanical and other physical properties of a composite material. However, CNTs tend to form a tangled mess resembling a hairball, which is difficult to work with. This and other difficulties have limited efforts toward realizing a composite material containing well-dispersed CNTs with preferred orientations.
Instead of trying to develop much lower-cost processes for making CNTs, researchers (Jang, et al.) at Nanotek Instruments, Inc. have worked diligently to develop alternative nano-scaled carbon materials that exhibit comparable properties, but are more readily available and at much lower costs. This development work has led to the discovery of processes for producing individual nano-scaled graphite planes (individual graphene sheets) and stacks of multiple nano-scaled graphene sheets, which are collectively called nano-sized graphene plates (NGPs). NGPs could provide unique opportunities for solid state scientists to study the structures and properties of nano carbon materials. The structures of these materials may be best visualized by making a longitudinal scission on the single-wall or multi-wall of a nano-tube along its tube axis direction and then flattening up the resulting sheet or plate (FIG. 1(a)). FIG. 1(b) shows an atomic force microscopic picture of a sample of NGPs. In practice, NGPs are obtained from a precursor material, such as minute graphite particles, using a low-cost process, but not via flattening of CNTs. These nano materials could potentially become cost-effective substitutes for CNTs or other types of nano-rods for various scientific and engineering applications.
Specifically, Jang, et al. disclosed a process to readily produce NGPs in large quantities [B. Z. Jang, L. X. Yang, S. C. Wong, and Y. J. Bai, “Process for Producing Nano-scaled Graphene Plates,” U.S. patent pending, Ser. No. 10/858,814 (Jun. 3, 2004)]. The process includes the following procedures: (1) providing a graphite powder containing fine graphite particles (particulates, short fiber segments, carbon whisker, graphitic nano-fibers, or combinations thereof) preferably with at least one dimension smaller than 200 μm (most preferably smaller than 1 μm); (2) exfoliating the graphite crystallites in these particles in such a manner that at least two graphene planes are either partially or fully separated from each other, and (3) mechanical attrition (e.g., ball milling) of the exfoliated particles to become nano-scaled, resulting in the formation of NGPs. The starting powder type and size, exfoliation conditions (e.g., intercalation chemical type and concentration, temperature cycles, and the mechanical attrition conditions (e.g., ball milling time and intensity) can be varied to generate, by design, various NGP materials with a wide range of graphene plate thickness, width and length values. Ball milling is known to be an effective process for mass-producing ultra-fine powder particles. The processing ease and the wide property ranges that can be achieved with NGP materials make them promising candidates for many important engineering applications. The electronic, thermal and mechanical properties of NGP materials are expected to be comparable to those of carbon nano-tubes; but NGP will be available at much lower costs and in larger quantities.
The NGP material can be used as a nano-scaled reinforcement for a matrix material to obtain a nanocomposite. Advantages of nano-scaled reinforcements in polymer matrices include: (1) when nano-scaled fillers are finely dispersed in the matrix, the tremendously high surface area could contribute to polymer chain confinement effects, possibly leading to a higher glass transition temperature, stiffness and strength; (2) nano-scaled fillers provide an extraordinarily zigzagging, tortuous diffusion path that results in enhanced barrier or resistance against permeation of moisture, oxygen, other gases, and liquid chemical agents; (3) nano-scaled fillers can also enhance the electrical and thermal conductivities in a polymer matrix; and (4) carbon-based nano-scaled fillers have excellent thermal protection properties and, if incorporated in a matrix material, could potentially eliminate the need for a thermal protective layer, for instance, in rocket motor applications. Enhancement in strength and stiffness for composites, to a great extent, depends upon the orientation of the reinforcement used.
In U.S. Pat. No. 6,934,600, issued on Aug. 25, 2005, Jang, et al disclosed a process of producing nanocomposite materials with oriented carbon nano-tubes (CNTs). Fundamentally, CNTs are viewed as one-dimensional nano-scaled reinforcements (needle-type). By contrast, NGPs are essentially two-dimensional nano-scaled reinforcements (platelet-type), which are expected to require substantially different processing methods or conditions to produce nanocomposites with a preferred NGP orientation.
It is to the provision of methods for producing composite materials containing well-dispersed NGPs with preferred orientations, and to composite materials containing well-dispersed NGP reinforcement with preferred orientations, that certain aspects of the invention are primarily directed.